Melons are normally direct seeded once all frost danger is past and the ground is dry enough to work and in your zone I'd guess that to be late April. You can start them indoors around 4 weeks earlier if you wish.

Eggplant and okra are both hot weather crops and require soil temps to be at least into the 70s before planting in the garden. 75 degrees is the ideal soil temp for them. So again, best guess is mid to late May. Start them inside if you wish 6-8 weeks earlier.

Hope this helps but you can get more accurate planting times for your specific area from your local country extension office and from any local gardeners.

Good luck.

Dave